The present invention relates to apparatus for measuring distance. Specifically, an apparatus and method are disclosed for determining the two-way transit time of an acoustic wave between a measuring position and reflective surface.
Measuring distances by calculating the transit time of a pulsed acoustic wave from a measuring position to a reflective surface has been known in the art. One class of such measuring devices includes an ultrasonic transmitter which directs a beam of ultrasonic energy to a distant surface. The two-way transit time for the ultrasonic burst to leave the transmitter site and return via reflection from a distant surface is measured and converted to a distance measurement. One of the difficulties in using this technique is that reflections from surfaces other than the surface of interest are received, and are difficult to distinguish from a main reflection from a distant surface of interest. Thus, the received reflection typically includes unwanted echoes which originate from surfaces other than the true surface whose distance is to be measured.
One technique for reducing this problem includes the use of reflective acoustic horn structures which define a narrow beam of ultrasonic energy, and which are highly directive, and therefore reduce the spurious side reflections returned with the pulse of interest. However, such horns which generally rely on a focussing effect are not advantageous due to their size and attendant difficulty in positioning.
Other limitations in determining distances using reflected ultrasonic energy include the change of velocity of the reflected wave as the temperature of the propagating medium changes. Thus, a technique for accurately measuring the ambient temperature of the medium in which the reflected energy travels should be employed to correct for changes in velocity. The velocity of ultrasonic energy propagation in air at 20 degrees Celsius and 50% relative humidity changes at a rate of approximately 0.18% per degree Celsius, necessitating accurate compensation if distance measurement precision is required.